The invention relates to dielectric resonator stabilized oscillators, and more particularly to a varactor circuit for electronically tuning the resonator.
Dielectric resonators have been used to stabilize the operating frequency of microwave oscillators. Because of the high Q of these resonators, the frequency of oscillation is essentially the same as the resonant frequency of the resonator. A common configuration of such oscillator employs an active circuit with a negative real part of impedance connected to an output transmission line. With no dielectric resonator, the impedance presented by the output transmission line is such that no oscillation occurs. When an appropriate resonator is coupled to the output line, it can present a proper impedance to the active circuit to cause oscillation. This occurs at the resonant frequency of the resonator. If the resonator Q is high, any tuning of the active circuit has little affect on the frequency of oscillation. About 0.1% or 0.2% tuning can be achieved by using a varactor junction diode in the active circuit. However, many applications require greater bandwidth.
It is known in the prior art to vary the resonant frequency of a dielectric resonator by coupling another resonant circuit containing a varactor diode to the resonator. A tuning bandwidth of over 1% has been achieved using this technique.
The present invention provides an improved varactor circuit for tuning the resonator.